Vehicular emissions which are the principle pollutants that have negative effects on public health and the natural environment are generally recognised to be carbon monoxide, hydrocarbons, nitrogen oxides (NOx) and particulate matter.
Diesel engines run at a high air to fuel ratio under very fuel lean conditions. Because of this they have low levels of emissions of gas phase hydrocarbons and carbon monoxide and are instead characterised by relatively high levels of emissions of NOx and particulate matter, relative to the current and agreed future emission regulations set by intergovernmental organisations. The control of particulate matter emissions and of NOx represent significant challenges to the diesel engine manufacturer because they are coupled inversely. Modern passenger vehicles include exhaust gas recirculation. When the engine operates cooler it produces less NOx but more particulate matter and conversely at higher temperatures combustion is more complete generating less particulate matter but more NOx. Therefore changes in engine design need to be combined with effective trapping and treatment processes to limit the emissions of these harmful pollutants to the atmosphere.
Emission legislation in Europe from 1st September 2014 (Euro 6) maintains the allowable limit set in Euro 5 (which came into force in September 2009 for the approval of vehicles and applied from January 2011 for registration and sales of new types of cars) for the mass of particulate matter emitted from diesel passenger cars of 4.5 mg/km as measured by the particulate measurement programme procedure.
However, for Euro 6, all vehicles equipped with a diesel engine will be required substantially to reduce their emissions of nitrogen oxides as soon as Euro 6 enters into force. For example emissions from passenger cars will be capped at 80 mg/km which is a reduction of more than 50% as compared to the Euro 5 standards. Furthermore combined emissions of hydrocarbons and nitrogen oxides from diesel vehicles will also be reduced. For example, these will be capped at 170 mg/km for passenger cars.
Therefore, the new Euro 6 emission standard presents a number of challenging design problems for meeting diesel emission standards. In particular, how to design a filter, or an exhaust system including a filter, to reduce the NOx and combined NO and hydrocarbon emissions, yet at the same time meeting the emission standards for PM pollutants and CO all at an acceptable back pressure, e.g. as measured by maximum on-cycle backpressure on the EU drive cycle.
Ambient particulate matter is typically divided into the following categories based on their aerodynamic diameter (the aerodynamic diameter is defined as the diameter of a 1 g/cm3 density sphere of the same settling velocity in air as the measured particle):
(i) Particles of an aerodynamic diameter of less than 10 μm (PM-10);
(ii) Fine particles of diameter below 2.5 μm (PM-2.5);
(iii) Ultrafine particles of diameter below 100 nm; and
(iv) Nanoparticles of diameter below 50 nm.
Since the mid-1990s, particle size distributions of particulates exhausted from internal combustion engines have received increasing attention due to possible adverse health effects of fine and ultrafine particles. Concentrations of PM-10 particulates in ambient air are regulated by law in the USA. A new, additional ambient air quality standard for PM-2.5 was introduced in the USA in 1997 as a result of health studies that indicated a strong correlation between human mortality and the concentration of fine particles below 2.5 μm.
Interest has now moved to consider ultrafine and nanoparticles generated by diesel and gasoline engines because they are understood to penetrate more deeply into human lungs than particulates of greater size and consequently they are believed to be more harmful than larger particles. This belief is extrapolated from the findings of studies into particulates in the 2.5-10.0 μm range.
Size distributions of diesel particulates have a well-established bimodal character that correspond to the particle nucleation and agglomeration mechanisms, with the corresponding particle types referred to as the nuclei mode and the accumulation mode respectively.
In the nuclei mode, diesel particulate is composed of numerous small particles holding very little mass. Nearly all nuclei mode diesel particulates have sizes of significantly less than 1 μm, i.e. they comprise a mixture of fine, ultrafine and nanoparticles. Nuclei mode particles are believed to be composed mostly of volatile condensates (hydrocarbons, sulphuric acid, nitric acid etc.) and contain little solid material, such as ash and carbon.
Accumulation mode particles are understood to comprise solids (carbon, metallic ash etc.) intermixed with condensates and adsorbed material (heavy hydrocarbons, sulfur species, nitrogen oxide derivatives etc.) Coarse mode particles are not believed to be generated in the diesel combustion process and may be formed through mechanisms such as deposition and subsequent re-entrainment of particulate material from the walls of an engine cylinder, exhaust system, or the particulate sampling system.
The composition of nucleating particles may change with engine operating conditions, environmental condition (particularly temperature and humidity), dilution and sampling system conditions. Laboratory work and theory have shown that most of the nuclei mode formation and growth occur in the low dilution ratio range. In this range, gas to particle conversion of volatile particle precursors, like heavy hydrocarbons and sulphuric acid, leads to simultaneous nucleation and growth of the nuclei mode and adsorption onto existing particles in the accumulation mode. Laboratory tests (see e.g. SAE 980525 and SAE 2001-01-0201) have shown that nuclei mode formation increases strongly with decreasing air dilution temperature but there is conflicting evidence on whether humidity has an influence.
Generally, low temperature, low dilution ratios, high humidity and long residence times favour nanoparticles formation and growth. Studies have shown that nanoparticles consist mainly of volatile material like heavy hydrocarbons and sulphuric acid with evidence of solid fraction only at very high loads.
Particulate collection of diesel particulates in a diesel particulate filter is based on the principle of separating gas-borne particulates from the gas phase using a porous barrier. Diesel particulate filters can be defined as deep-bed filters and/or surface-type filters. In deep-bed filters, the mean pore size of filter media is bigger than the mean diameter of collected particles. The particles are deposited on the media through a combination of depth filtration mechanisms, including diffusional deposition (Brownian motion), inertial deposition (impaction) and flow-line interception (Brownian motion or inertia).
In surface-type filters, the pore diameter of the filter media is less than the diameter of the particulate matter, so particulate matter is separated by sieving. Separation is done by a build-up of collected diesel particulate matter itself, which build-up is commonly referred to as “filtration cake” and the process as “cake filtration”.
It is understood that diesel particulate filters, such as ceramic wallflow monoliths, may work through a combination of depth and surface filtration: a filtration cake develops at higher soot loads when the depth filtration capacity is saturated and a particulate layer starts covering the filtration surface. Depth filtration is characterized by somewhat lower filtration efficiency and lower pressure drop than the cake filtration.
Diesel particulate filters have been shown to be extremely effective at removal of particulate matter over the entire particle size range. However these filters have limited capacity for trapping particulate matter before the pressure-drop becomes excessive therefore it is necessary periodically to regenerate the diesel particulate filter. Passive regeneration does not readily take place as combustion of the retained particulate matter in the presence of oxygen requires higher temperatures than those typically provided by diesel engine exhaust. One effective method to lower the combustion temperature of the trapped particulate matter on the diesel particulate filter is addition of a catalysed washcoat to the filter wall. Compositions of catalysed washcoats used are similar to those used in diesel oxidation catalysts and typically comprise at least one platinum group metal. The reactions on the catalysed diesel particulate filter include oxidation of CO and HC and oxidation of NO to NO2 which enables combustion of the particulate matter at a much lower temperature than in the presence of oxygen.
U.S. Pat. No. 7,722,829 discloses a catalysed soot filter which simultaneously treats the CO and HC gaseous components and the particulate matter in diesel exhaust gas. The diesel particulate filter is a wall flow substrate which is coated in a washcoat comprising a platinum group metal. The washcoat is coated on to part of the internal walls of both the inlet and outlet passages of the substrate such that more than 50% of the platinum group metal components are present on the walls of the inlet passages. US '829 discloses that the positioning of the majority of the platinum group metal components upstream was found to improve the efficiency of the platinum group usage and allow the substrate to maintain its catalytic function even after multiple regeneration cycles. Furthermore it discloses that this positioning was also found to favour the combustion of soot and regeneration of the filter with the following reasoning. The high concentrations of platinum group metals in the upstream zone of the substrate generated increased concentrations of NO2 in the upstream zone (by oxidation of NO present in the diesel engine emissions) which could flow towards the outlet passages to combust soot deposited in the downstream zone where most of the soot was collected. US '829 discloses that the coatings may be disposed as a thin coating on the surface of the internal walls of the wall flow substrate and/or may permeate the porous walls to some extent.
A variety of technologies have been explored to reduce NOx emitted from diesel exhaust systems to environmentally acceptable nitrogen for release to the atmosphere. Selective NOx reduction (lean NOx catalyst) using the on-board diesel fuel or a derivative to selectively catalyse the oxidation of HC and NOx to CO2, H2O and N2 was extensively investigated and two main candidate materials identified as selective catalysts. However it has been reported in the literature that it is thought that this system will not be sufficient to meet the stringent requirements of Euro 6.
Lean NOx traps (NOx adsorber catalyst) use a basic metal oxide to adsorb NOx during the lean mode of operation. Exhaust gas rich in NO is converted to NO2 over a platinum group metal-containing catalyst and the NO2 is trapped and stored on e.g. an alkaline metal oxide which is incorporated within the platinum group metal-containing catalyst. The NO2 is then desorbed under rich conditions and reduced using rhodium which is also incorporated on the catalyst.
SCR involves use of ammonia in the presence of a suitable catalyst, which ammonia acts as a selective reductant for NOx. Typically urea is the source of the ammonia, which hydrolyses in the exhaust system at about 200° C. Suitable catalysts include metal exchanged zeolites and mixed catalysts of vanadium and titanium dioxides. The technology is potentially capable of NOx reduction of greater than 90% so it is seen as a good candidate for meeting the new stringent NOx requirements for diesel engines. However the SCR is prone to contamination from HC, CO and particulates which reduces its effectiveness. Furthermore, for many diesel engines, a majority of NOx emitted from the exhaust system is in the form of NO, whereas a faster SCR reaction proceeds from a mixture of NO and NO2. NO2 is a more reactive compound than NO and the faster SCR reaction can extend the operating temperature of the SCR process to lower temperatures.
WO 02/14657 discloses an aftertreatment system for lean burn diesel applications configured with a catalysed soot filter upstream of a zeolite SCR to produce substantially better NOx conversion performance than the SCR catalyst alone. The catalysed soot filter is coated on the internal walls of the filter substrate with the catalyst being applied by solution impregnation. This application technique suggests that the catalyst is substantially present within the pores of the internal walls of the substrate to minimise increase in exhaust gas back pressure caused by the catalyst as far as possible. The catalyst is coated along the full length of both the inlet channel internal walls and the outlet channel internal walls of the filter substrate. It is mentioned at page 31, lines 28-29 that it may be possible selectively to coat portions of the channels but no further exemplification is provided.
We have now designed a new catalysed soot filter for treatment of diesel engine exhaust emissions which removes PM, HC and CO from the exhaust emissions and simultaneously enriches NO2 concentration in NOx emitted from a diesel engine to enable more efficient treatment of NOx, for example using a SCR catalyst. The catalysed soot filter is designed such that the catalyst is substantially coated on, not in, the internal walls of the filter substrate, with different axial coating lengths on the inlet channels and outlet channels, which arrangement has been found to maintain an acceptable exhaust gas back pressure with respect to coating loading and soot trapping and regeneration and has also been found to provide enhanced NO2 enrichment as compared to filters with coatings substantially or partially “in-wall”.